1. Field of the Invention
This invention relates to improved electric power steering equipment wherein additional steering torque is generated by an electric motor to aid the driver of a vehicle in a steering operation, and more particularly to control means for controlling drive of the electric motor to generate proper additional torque thereby to provide a constant steering feeling to the drive regardless of various running conditions of the vehicle.
2. Description of the Prior Art
Power steering equipment has been known as a means for helping a driver to steer an automobile. This power steering equipment generates a driving force in accordance with the steering operation by the driver and transmits the driving force thus generated to a steering gear. Most power steering equipment put to practical use at the present time is actuated by a hydraulic drive. That is, the hydraulic drive for the power steering equipment is provided with a control valve, a hydraulic cylinder and associated components to generate an additional steering torque by moving oil within the cylinder in accordance with the steering operation.
However, the control valve, the hydraulic cylinder and so forth are large in size and therefore a disadvantage is encountered such as large pressure loss within pipes connecting them. To prevent this pressure loss, the pipes must not be bent with a curvature of less than a predetermined curvature. In the hydraulic drive, moreover, a seal must also be securely effected so as not to leak oil. Further, handling of the equipment at the time of installation thereof is difficult. Therefore, in the case of a vehicle not having enough mounting space in the engine compartment, such as a front wheel drive vehicle, it is further difficult to mount the power steering equipment therein. Therefore, it becomes preferable to use an electric motor as a driving device for the power steering equipment. In this case, various needed controls for the equipment can be easily performed.
Generally, the torque required for the steering operation varies inversely with the vehicle's speed. Namely, when the vehicle speed is low, large steering torque is required for the steering operation. In contrast, as the vehicle speed increases, it is possible to operate the steering wheel with smaller torque input. For a vehicle not having power steering equipment, when it is in a stopped state or in a low speed running state, large steering torque is required to operate the steering wheel of the vehicle. In contrast, with power steering equipment it is possible for the driver to operate the steering wheel with relatively small torque input.
On the other hand, if the power steering equipment is operated at a time when the vehicle is running at high speed, it is possible for the steering torque required of a driver to excessively decrease. Thus, if the steering torque required of a driver is decreased at the high speed running state of the vehicle, there is a possibility that the driver will excessively rotate the steering wheel due to inexperience with the power steering equipment.
Accordingly, the present invention proposes electric power steering equipment in which additional steering torque is at least controlled in accordance with the vehicle speed.
However, even if the vehicle speed is constant, the total torque required to actually steer an automobile changes due to various factors other than the vehicle speed. Particularly, the aforesaid torque greatly changes according to roadway conditions such as wetness on the road. Generally speaking, the coefficient of sliding friction of tires on a road changes by a degree of 0.2 when comparing a dry road condition with a wet road condition. Thus, the total torque required to steer also greatly changes in accordance with changes of the aforesaid coefficient of sliding friction. Therefore, a driver's steering feeling also greatly changes with road conditions before and after raining, even when the veicle is running at a constant speed on the same road. In other words, if the servo ratio is constant regardless of the vehicle's speed or the wetness of the road surface, manual steering torque to be applied to a steering shaft by a driver varies with respect to the vehicle speed or wetness. Therefore, the torque input is so light for the driver to steer at either high speed or high wetness that oversteering may well occur.